Thermally stable cellulose products



United States Patent ABSTRACT OF THE DISCLOSURE A thermally stabilized cellulosic insulation material for use in a transformer in the presence of air and a liquid dielectric and comprising a cellulosic paper having from about 0.02% to about by weight, based on the total weight of the cellulosic paper, of at least one nitrogencontaining stabilizer selected from the group consisting of: acetamide, bis(hexarnethylene) triamine, caffeine, dimethylglyoxime, hexamethylene phosphoric triamide, phthalimide, N-nitrosodiphenylamine, and trimethylamine borane.

This invention relates to electrical apparatus embodying cellulosic insulation and, in particular to stabilizing cellulosic insulation in \contact with liquid dielectrics.

Cellulosic materials such as paper, cotton cloth, cotton tape, pressboard, and wood deteriorate relatively rapidly at temperatures in excess of 100 C. when in contact with air. Deterioration is even faster at such elevated temperatures when such materials are in contact with liquid dielectrics such as transformed oil, especially when the oil is permitted to be oxidized.

For this reason, electrical apparatus employing cellulosic insulation generally is not operated continually at temperatures above 105 C.

One of the factors involved in determining the maximum temperature at which electrical apparatus may be operated safely is the ability of the cellulosic insulation to retain appreciable mechanical and electrical strength while being operated at such maximum temperatures. This limitation applies to cellulosic insulation in contact with or immersed in liquid dielectric though it is a problem when cellulosic insulation is heated in contact with air.

Patent No. 2,722,561 discloses that substances like urea or non-acidic compounds derived from urea, when added to oil filled transformers, greatly improve the thermal life of the cellulosic insulation contained therein. The patent broadly discloses that the addition of about 3% of urea (or one of its equivalents) to the oil, based on the. total weight of the oil, provides paper having about the same life at 125 C. as it has at 100 C. without the urea stabilizer being present. The life of the cellulosic material is measured in terms of retention of tensile strength and crease resistance.

In accordance with this invention, it has been found that certain compounds can be incorporated in cellulosic materials to enhance their resistance to deterioration at even higher temperatures.

Accordingly, it is a general object of this invention to provide a thermally stable cellulosic product for use as insulation in contact with liquid dielectrics by incorporating in the cellulosic material certain stabilizing compounds.

It is a further object of this invention to satisfy the foregoing probems and desiderata in a simple and ef fective manner.

Briefly, the device of the present invention comprises a thermally stabilized cellulosic insulation material for use in a transformer in the presence of air and a liquid dielectric, which material is stabilized against loss of physical strength with passage of time at elevated temperatures, the material comprising a cellulosic paper having from about 0.02% to about 10% by weight, based on the total weight of the cellulosic paper, of at least one nitrogen containing stabilizer selected from the group consisting of: acetamide, bis(hexamethylene) triamine, calfeine, dirnethylglyoxime, hexamethylene phosphoric triamide, phthalimide, N-nitrosodiphenylamine, and trimethylamine borane. The preferred amount of stabilizer is from 0.5 to 5%. The stabilizers are preferably distributed throughout the interstices within the cellulosic paper by being incorporated in the paper fibers during the process of making the paper. However, the stabilizers can be added to paper, cloth or pressboard by soaking them in a solution of the stabilizer, and drying to remove the solvent which latter may be an alcohol, water, or other volatile liquid.

The electrical insulating and physical properties of cellulosic material deteriorate rapidly at temperatures above C. when in cont-act with air or fluid dielectric compositions. Thus, for example, after being immersed for only a few weeks in transformer oil at to C., kraft paper will retain only a few percent of its original dielectric strength and practically none of its original tensile strength. Electrical grade kraft paper may be bent or flexed several hundred times before it breaks. However, after only a few weeks immersion in transformer oil at 150 C. it will break upon being folded on itself only one time. For these reasons, it has been generally specified in the industry that, in electrical apparatus employing cellulosic insulation, the continuous operating temperature shall not exceed about 105 C.

In accordance with the present invention, it is now possible to greatly reduce the rate of degradation and to increase the retention of both dielectric strength and mechanical strength of cellulosic insulation at elevated temperatures by substantially uniformly distributing throughout the insulation effective amounts of the above nitrogen containing chemical stabilizing compounds. Although the amount of the compound present may be small, it nevertheless imparts a highly beneficial stabilizing effect to the electrical insulation.

The nitrogen containing chemical compounds which have been found to impart these improvements are: acetamide, bis(hexarnethylene) triamine, caffeine, dimethylglyoxime, hexamethylene phosphoric triamide, phthalimide, N-nitrosodiphenylamine, and trimethylamine borane. Two or more of these compounds may be employed simultaneously in stabilizing cellulose. Generally, these compounds exhibit substantial solubility in water or water and alcohol solutions which considerably enhances their incorporation into cellulosic insulation especially from the economic standpoint. Moreover, these compounds exhibit substantial insolubility in refined petroleum oil which facilitates their use in an electrical apparatus employing any refined petroleum oil liquid dielectric;

The benefits of the additives of this invention are dependent upon several factors. First, the stabilization compounds must be present in the cellulosic insulation in amounts within the range of about 0.02% to about 10% by weight based on the weight of the cellulosic material. Less than 0.02% stabilization compounds does not impart to the insulation any appreciable improvement in either electrical insulation or mechanical strength at elevated temperatures. The presence of more than about 10% of the compounds is both diflicult to produce, it is uneconomical and does not appreciably increase the degree of improvement beyond that obtained with 10%. Within this broad critical range, it is preferred to incorporate in the cellulose about 0.5% to about 5% of the stabilization compounds, inasmuch as these amounts having been found to impart the optimum desired improvements inu the electrical insulating and thermal stability properties of the cellulosic insulation.

Second, the stabilizing compound or compounds, if more than one is used, must be present in substantially uniform distribution, that is being intimately present throughout the interstices of the fibers comprising the cellulosic insulation, to obtain optimum benefits. This requirement is readily satisfied because of the fact that (1) all of the members of the group of stabilizing compounds of the invention are substantially soluble in water or water alcohol thus facilitating uniform and thorough incorporation into the cellulosic insulation from such solutions, and (2) these compounds are substantially oil insoluble thus preventing depletion of the uniform distribution of the stabilization compound in the cellulosic insulation in the presence of oil type dielectrics.

Where, for example, the stabilizing materials are merely suspended in a liquid dielectric, an extended period of time elapses before the stabilizers are carried in any Significant amount to permeate the cellulosic insulation so as to function at substantial effectiveness. Inasmuch as all of the members of the stabilizing compounds of this invention are soluble in water or water alcohol mixtures, they may be desirably incorporated in the insulation during its manufacture or fabrication. In the case of paper insulation particularly incorporation of the compound may be made in the paper mill. Paper is generally made on either a Fourdrinier machine or a cylinder type machine. With either method, the formed Web of felted cellulosic fiber is transferred from the forming screen to a felt belt for drying. During drying, the web passes between calender rolls to impart a particular surface finish or density after which is it stored for shipment.

In practicing the invention with respect to paper insulation the stabilizing compounds in substantially aqueous solution are added to the partially dried paper which absorbs a predetermined amount of the stabilizing compounds. The usual solution temperature is about 60 to 90 C. to produce a suitable concentrated solution. After this treatment the paper passes through a second portion of the drier.

In order to more fully describe some of the benefits obtained by practicing the invention, reference should be had to the table which lists mechanical strength retention of treated paper for the stabilization compounds of the invention. The table also includes the mechanical strength retention for untreated kraft paper for comparison purposes. In each case, 3% by weight of the particular stabilizing compound was added to the kraft paper during its manufacture. In most instances, the paper was about 5 mils thick and had a density of approximately 1. Each of the samples of paper was wound with enameled wire into a coil and sealed in a vessel filled with transformer core iron being also placed in the vessel. Sufficient current was circulated through the coil to generate temperatures of about 140 C. The coil unit was removed after 7 days and a Mullen bursting strength test run on the aged paper. The table lists the percent strength retention of the aged samples as compared to the Mullen bursting strength of the fresh or unaged kraft paper. Each value represents an average of at least 5 tests.

TABLE Stabilizing compound: Percent retention Acetamide 73 Bis(hexamethylene) triamine 79 Caffeine 83 Dimethylglyoxime 74 Hexamethylene phosphoric triamide 83 Phthalimide 77 N-nitrosodiphenylamine 75 Trimethylamine borane 77 Untreated kraft paper 26 There is a direct relation between mechanical strength retention and dielectric properties retention for the cellulosic materials of the invention in that where the treated cellulosic material exhibits a high retention of mechanical strength such as the treated paper indicated in the table, it also exhibits correspondingly high retention of dielectric integrity.

It is to be noted that with regard to the application of the invention to transformers, the transformer construction may be more solid and tighter because of the treated cellulosic spacers, and other components will lose less than half the thickness loss on prolonged thermal aging exhibited by untreated pressboard, kraft paper, or other cellulosic materials.

It has also been found that where certain properties are desired, such as higher initial tensile strength of cellulosic insulation and water proofness, that certain resins can be incorporated in the paper in the process of beating the pulp. These resins can be introduced in finely divided or emulsified form in the beating of the pulp or they can be introduced later from organic solution. Small amounts of up to 5% of a resinous material such as phenolic, epoxy, acrylic or diallylphthalate resins have been found to be compatible with the stabilizing compounds of the invention and further enhance mechanical, electrical and thermal stability of the finished product. This 1% of polyacrylamide resin or 2% of an epoxy resin added as a suspension to paper pulp produces an improved paper in which the additives may be incorporated.

It is to be understood that the above description is illustrative and not in limitation of the invention.

What is claimed is:

1. A thermally stabilized cellulosic insulation material for use in a transformer in the presence of air and liquid dielectrics which material is stabilized against loss of physical strength with passage of time at elevated temperatures, the material comprising a cellulosic paper having from about 0.02% to about 10% by weight, based on the total weight of the cellulosic paper, of at least one stabilizer selected from the group consisting of acetamide, bis(hexamethylene) triamine, dimethylglyoxime, hexamethylene phosphoric triamide, phthalimide, caffeine, N-nitrosodiphenylamine, and trimethylamine borane, said stabilizers being distributed throughout the interstices within the cellulosic paper.

2. The cellulosic insulation material of claim 1 in which the amount of stabilizing compounds varies from about 0.5% to about 5% by weight.

3. The insulation of claim 1, wherein the cellulosic material incorporates up to 5% of a resin to provide for improved mechanical, electrical insulating and thermal resistance properties of the paper.

4. The cellulosic insulation material of claim 1 in which the stabilizing compound comprises phthalimide.

5. The cellulosic insulation material of claim 1 in which the stabilizing compound comprises N-nitrosodiphenylamlne.

6. The cellulosic insulation material of claim 1 in which the stabilizing compound comprises caffeine.

7. The cellulosic insulation material of claim 1 in which the stabilizing compound comprises hexamethylene phosphoric triamide.

References Cited UNITED STATES PATENTS 2,787,516 4/1957 Compton et al. 8116.2 XR 2,793,930 5/1957 Compton et al. 81l6.2 XR 3,211,516 10/1965 Sadler 81l6.2 3,316,178 4/1967 Millington 252-63.7

LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

U.S. Cl. X.R.

8-ll6.2; 162138, 2602l2 

